US 20050026044 A1
An electrolyte for a battery comprises LiBOB salt in gamma butyrolactone and a low viscosity solvent. The low viscosity solvent may comprise a nitrile, an ether, a linear carbonate, or a linear ester. This electrolyte is suitable for use in lithium ion batteries having graphite negative electrodes. Batteries using this electrolyte have high conductivity, low polarization, and high discharge capacity.
51. An electrolyte consisting of:
one or more salts, including LiBOB;
one or more lactones; and
one or more low viscosity solvents; wherein
the electrolyte is capable of forming an effective SEI layer on an electrode.
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72. A lithium battery comprising:
a battery case;
an electrode assembly housed in the case; and
an electrolyte in the case, the electrolyte including one or more lactones, one or more low viscosity solvents, one or more salts, and being capable of forming an effective SEI layer on an electrode; wherein the one or more salts includes LiBOB.
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92. An implantable medical device including the battery of
93. A method for making a battery comprising the steps of:
providing a battery case;
housing an electrode assembly within the battery case; and
inserting an electrolyte into the battery case, the electrolyte including one or more lactones, one or more low viscosity solvents, one or more salts, and being capable of forming an effective SEI layer on an electrode; wherein the one or more salts includes LiBOB.
This application claims the benefit of U.S. Provisional Application No. 60/408,100 filed Sep. 3, 2002.
This invention relates to an electrolyte and more particularly to an electrolyte for use in a battery.
An effective solid electrolyte layer (SEI) must be created at the surface of a graphite negative electrode of a battery in order to keep the electrolyte from decomposing. Various electrolytes comprising certain combinations of salts and solvents produce SEI layers of various qualities. Typical lithium ion batteries use an electrolyte comprising LiPF6 in a carbonate solvent, with 1.2-M LiPF6 in ethylene carbonate (EC): diethyl carbonate (DEC) being typical in the battery industry. EC is solid at room temperature and requires additional processing steps for employing in an electrolyte. Graphite electrodes have a fragile structure and, until the invention of the electrolyte described herein, have required the use of EC for forming the SEI layer without damaging the graphite structure. By contrast, hard carbon negative electrodes are not as easily broken and therefore can use solvents other than EC to form the SEI layer. However, while hard carbon has a higher capacity than graphite, it can absorb a lot of moisture and has a large irreversible capacity, making graphite a much more desirable electrode material than hard carbon. Lithium metal does not require EC to form an SEI layer, but is useful only for a primary battery, not rechargeable. Vinylene carbonate (VC) and vinyl ethylene carbonate (VEC) can aid in creating an SEI layer, but can only be used in quantities up to about 3% because an excess of these solvents creates degradation at the positive electrode; with this small quantity of SEI-forming solvent, only a thin SEI layer is created, with all of the VC or VEC consumed during the first charging cycle; therefore, another SEI-forming component such as EC must be added.
The electrolyte of the present invention comprises a salt or mixture of salts comprising lithium bis(oxalato) borate (LiBOB) in a lactone solvent or mixture of lactone solvents, preferably gamma-butyrolactone (GBL), combined with a low viscosity solvent or mixture of low viscosity solvents, and preferably does not contain a solvent that is solid at room temperature, such as ethylene carbonate (EC). This inventive electrolyte is useful in primary and secondary batteries, and is especially suitable for a lithium ion battery having a graphite negative electrode, forming a functional SEI layer that does not readily decompose.
LiBOB is more soluble in lactone solvents, such as gamma-butyrolactone (GBL), than in commonly used carbonate solvents, such as ethylene carbonate (EC) and propylene carbonate (PC). Using a lactone solvent to dissolve LiBOB electrolyte produces a high salt concentration electrolyte, greatly improving conductivity as compared with using a carbonate solvent.
This electrolyte system has a wide operating temperature range and therefore can be safely used in many applications, including satellites and implantable medical devices. For example, a high temperature sterilization process could not be used for many electrolytes; the salt LiPF6 decomposes at about 80° C., and DEC boils at about 126° C. By contrast, LiBOB is stable at 300° C., and GBL boils at about 206° C., making this combination ideal for high temperature sterilization. At the other temperature extreme, EC has poor low temperature performance due to its high freezing point of around 37-39° C., making it very viscous at low temperatures, and therefore less desirable for applications in which low temperature operation is important.
Furthermore, in the case of a leak, unlike fluorine-containing salts such as LiPF6, LiBOB does not form HF when mixed with bodily fluid, and is therefore safer than LiPF6. While LiBF4 decomposes at a lower rate than LiPF6 and is therefore slower to form HF, it has lower conductivity than LiPF6 due to its lower dissociation.
The following text describes the preferred mode presently contemplated for carrying out the invention and is not intended to describe all possible modifications and variations consistent with the spirit and purpose of the invention. The scope of the invention should be determined with reference to the claims.
The electrolyte of the present invention is a solution of LiBOB salt, a low viscosity solvent, and a lactone, for example, gamma-butyrolactone (GBL).
A typical electrolyte comprises 1.2-M LiPF6 in EC:DEC. The viscosity of EC is about 1.86 centipoise (cP) at 40° C. GBL has a viscosity of about 1.7 cP at room temperature. A low viscosity solvent is one that will lower the overall viscosity of the electrolyte comprising LiBOB and GBL and is therefore less viscous than GBL. Therefore, the low viscosity solvent itself has a viscosity of less than about 1.7 cP and more preferably less than about 1 cP. Low viscosity solvents can be chosen from among the following: nitrites such as acetonitrile, ether such as dimethyl ether (DME) or tetrahydrofuran (THF), linear carbonates such as diethyl carbonate (DEC) and methyl ethyl carbonate (MEC), and linear esters such as propyl acetate (PA) and methyl acetate (MA). An advantage of using a noncarbonate low viscosity electrolyte is that carbonates tend generate CO2 gas when decomposing, which can cause the battery to swell.
An electrolyte of the present invention may be made simply by combining a measured mass of GBL with a measured mass of low viscosity solvent, such as PA, then dissolving in a measured mass of LiBOB salt. The entire process may be completed at room temperature, or even lower, if desired.
By contrast, an electrolyte containing EC requires first melting the EC at elevated temperature such as in an oven in a dry environment, which can take about 5 hours for a 1-L bottle. Then the melted EC must be transferred immediately to an argon box and accurately weighed. Then it must be quickly combined with one or more additional weighed solvents, and then the measured mass salt dissolved before the EC begins to recrystallize. Because of the additional steps of melting the EC and the required use of heat, manufacturing an EC-containing electrolyte is more expensive than manufacturing the electrolyte of the present invention. Scaling up the EC-containing electrolyte manufacturing process is costly, requiring expensive equipment.
A battery of the present invention may be made by housing an electrode assembly in a battery case and inserting an electrolyte as described herein into the case, wherein the electrolyte comprises LiBOB salt in a combined solvent of lactone, preferably GBL, and a low viscosity solvent. The negative electrode of the electrode assembly may comprise graphite, hard carbon, lithium, lithium alloy, SiO, Si, SnO, Sn, and/or any other negative electrode material known in the art. The negative electrode may further comprise a negative electrode substrate made of copper, titanium, nickel, or stainless steel. The positive electrode may comprise a carbon fluoride, a cobalt oxide, a nickel oxide, a nickel cobalt oxide, a manganese oxide, a manganese cobalt oxide, a nickel cobalt manganese oxide, silver vanadium oxide (SVO), a lithium titanium oxide, iodine, and/or any other positive electrode material known in the art. The positive electrode may further comprise a positive electrode substrate made of aluminum, nickel, titanium, or stainless steel. The battery may be a primary or secondary (rechargeable) battery. If it is a rechargeable battery, it may be a lithium ion battery having a liquid electrolyte, or may have a polymer electrolyte, which could be a gel or a solid in combination with a liquid electrolyte. For an implantable medical device, the device housing and/or the battery, which may be housed within the device housing, is hermetically sealed. For a medical device requiring high temperature sterilization or for other high temperature applications, the low viscosity solvent is preferably chosen to have a high boiling point, such as greater than 126° C.
While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims. Furthermore, various aspects of the invention may be used in other applications than those for which they were specifically described herein.